Monday, March 14, 2011

Corium

Hopefully we do not start hearing about corium in the news coming out of Japan. Here is a little primer on the stuff in case mention of it does somehow start to creep into newscasts.

Corium is the lava-like material formed from the melted core of a nuclear reactor. What happens to this material determines the extent of environmental contamination a meltdown causes. Corium is not much of an environmental problem if it stays in the reactor containment vessel (assuming there is a containment vessel; there was not one in the Chernobyl plant). Corium can cause big problems if it escapes from the reactor vessel, and it is very difficult to contain molten corium.

Corium, since it is formed from a critical mass of nuclear fuel, can form a critical mass itself. It is possible in a pressurized water reactor (or boiling water reactor) for a “prompt critical” mass of fuel material to collect at the bottom of a reactor vessel. This would be a worst-case scenario, and highly unlikely.

A much more likely scenario is one where the nuclear fuel material mixes with all sorts of other material as the molten material drips out of the core. It is possible that, since molten corium should be rather viscous, there are unmixed zones in the corium blob that are still locally critical.

The reason criticality is a very important concern is that a critical corium blob continues to generate heat until it is dispersed. Because there is no mixing of the corium blob unless it moves the heat would likely build up until some critical failure moved the blob. In the case of a “prompt critical” blob the failure might even be vaporization of portions of the blob and containment structures. The resultant plume would cause a catastrophic environmental disaster. For the locally critical blob the movement would likely come in the form of a containment vessel breach; the blob would be dispersed as it splashed onto the containment building’s floor.

It does not require a locally critical mass to breach a containment vessel and create a corium splash. However, with a large enough locally critical nuclear fuel zone in the corium blob the containment vessel breach is almost inevitable. A blob without a locally critical zone is the most likely scenario.

In the most likely scenario, and the only scenario observed in any nuclear disaster to date, the blob cools as it dissipates the heat generated while it was critical. The number of neutrons the mass generates decreases, and the sub-critical mass begins to act like a lump of hot metal. The famous corium flow at Chernobyl called “the elephant’s foot” is now only slightly warmer than ambient temperatures, and it has only been 25 years since that was formed. It is important, however, to remember that the Chernobyl corium flow was many metric tons in size (the elephant’s foot alone was two metric tons) and so the fuel was diluted far beyond the concentration where any sub-critical nuclear reactions would contribute significantly to its heat.

The hot metal is very hot even for hot metal. When it comes into contact with normally non-volatile material, like concrete, the outgassing can cause explosive dispersal. The amount of dispersal is dependent on the amount of heat in the corium, and the outgassing potential of the material it comes into contact with. The dispersal energy would determine the size and scope of the environmental disaster.

The other contributing factor would be the concentration of radioactive material in the ejecta. Most of the material in the corium blob would either be highly radioactive before entering into the blob, or become highly radioactive because it adsorbed neutrons from fuel material fission within the blob. Other material would become radioactive if it were dispersed with splatters from the blob.

If corium is explosively dispersed it could become a very widespread problem. Corium is so hot that many materials interacting with it are melted into a glasslike or ceramic state. Much of this material is naturally friable. In addition to natural friability the highly radioactive material spontaneously degenerates, causing small-particle generating fractures. Even large chunks of ejecta can form small respiratable particles which easily disperse over enormous areas, or re-aerosolize.

Corium is scary stuff, but please turn off your nightlight. Conserving energy is much safer than building any type of reactor. And who needs nightlights these days anyway?

I’ll look at the discussion. If I can be of use I’d be happy to contribute.

One of the questions often being answered is “why don’t meltdowns go supercritical?” ie. “Why don’t they create a nuclear explosion?” A good understanding of why they don’t provides a good framework to understand why corium should not be critical, and why subcritical can be fairly bad.

Critical mass is more a description of neutron flux than actual mass of material. Perhaps a better term would be critical concentration, but I think “critical mass” sounds better; don’t you? In the supercritical mass the fuel can act as moderator and neutron generator. Fission produces mostly fast neutrons which are not absorbed at high probability by fuel nuclei, but if you have enough fuel nuclei this problem is obviated. Some nuclei will absorb fast neutrons and give off slow neutrons, and a gamma ray. Some of the first reactor designs were slurry reactors where finely ground moderator and fuel were mixed together. I suppose if a slurry reactor melted down the corium would be critical. Modern reactors use fuel rods and both solid and liquid moderators. The fuel in the rods has both an internal and a surface neutron flux. The perfect nuclear reactor would have surface flux where all the fast neutrons escaping the fuel rod were reflected back as slow neutrons. This would allow the fuel to “burn” most efficiently. By arraigning the fuel rods and moderators in an optimal configuration the closes to perfect flux is achieved. If you remove the moderator the fuel is not critical by itself. If you change the arrangement the fuel should go subcritical also. If it were not for the fact that much more fuel is present for the minimal critical configuration it would be so statistically unlikely to even have local areas of critical flux that it would really be impossible. With the overabundance of fuel it is only highly improbable that a local slurry reactor zone could form in the corium blob.

Subcritical neutron flux does not mean that fission has stopped; it only means that fission will eventually drop to the level expected with only spontaneous fission. Depending on how subcritical the reactor is this could take a while.

I get the general idea of the significance of concentration, however doesn't sheer mass also matter? As you note "if you have enough fuel nuclei this problem is obviated" which implies that even with the same density a larger mass will create a larger cascade of nuclear reactions. In other words, wouldn't the critical density be decreased as the mass increases? What is important is the total probability that a neutron will induce another neutron to be released.

Since some spent fuel pools now contain over 1000 tons of fuel is it possible that if these pools melted down that a critical density X mass (mass-density?) could exist.

I know that the larger pools are made up of only the older, cooler fuels but I got the impression from a Sciam article that only a small percentage of the energy in the rods is actually expended before the rods are considered 'spent'.

Without a minimum mass criticality cannot be achieved. So, yes, sheer mass is vitally important. It is possible to reach criticality with certain subcritical masses just by adding material. Yes, modeling the fate of neutrons as a probability is an optimal way of determining configurations that will go critical.

I would go further. In beginning Nuclear Engineering classes (and it has been decades since I took mine) one of the first types of reactors that is modeled is the homogeneous infinite core. This is a model similar to what one might use to predict criticality of corium. A neutron has a chance (based on core composition) of destructive, constructive, or neutral reactions. The neutral reactions can include ballistic and non-ballistic slowing interactions, and the resulting slow neutrons have a different set of probabilities for destructive, constructive, and neutral reactions. I’m avoiding looking up the differential equations for this because if I did I would be tempted to write a blog entry on them, and people would complain that I am being an obtuse intellectual elitist (that and I’ve only had one cup of coffee this morning). So I’m trying to describe this without the equations which can make this sound a bit ephemeral.

Fuel that can go supercritical is almost two-orders of magnitude more concentrated than power plant fuel. Most of the difference is in the concentration of U238 (for plutonium reactors the names are changed, but the basic ideas are the same) which has a large destructive neutron capture cross section (and the resulting U239 decays into P239, which is the principle behind breading) so it acts as a neutron poison. This means two things: that you need a much larger mass for criticality in the power plant fuel, and that the probability of generating a slow neutron and then having it constructively interact is small (this is the contribution of the internal neutron flux of the fuel rods). In order for the fuel rods to burn you have to supply either some moderated neutrons or a whole bunch of fast neutrons from outside. If you bundle up a bunch of fuel rods you will need fewer to reach criticality if you put the bundle in a bath of moderator.

Can you melt a bunch of fuel rods into a critical mass? Yes you can. Is there enough in a reactor? Yes, there is more than enough. In TMI something like 10 tons of material melted down. It is, however, very improbable.

In addition to the moderator and fuel in a SCRAMed reactor there are structural components, control rods, and –presumably- other neutron poisons (usually boron). The intense heat would also melt these materials. Mixed together in close proximity within the corium poisons would be more effective in shutting down critical neutron flux (it might be worthwhile to point out that the destructive neutron interactions usually produce radioactively unstable nuclei).

The idea (which I have not purposely tried to confuse) is that the concentration of fissile nuclei, moderator nuclei, and poison nuclei determine the neutron flux. With power plant fuel it is difficult to get a geometry that achieves that balance. In an uncontrolled meltdown the series of improbable partial melting needed to do this would be highly improbable. If you want a more specific discussion of the geometries of the TEPCO reactors and how these could contribute to the probabilities then you will need to get in touch with an engineer who is much more familiar with them than I.

The spent fuel storage location cannot go critical without significant outside intervention, but going critical is not the big worry with them. First the temperature is way too low to alter the geometry in such a way as to produce a significant corium blob. Second, most of the fission byproducts can act as neutron poisons. In order to recover the “unburnt” nuclear fuel the rod material must be cleaned of the poisons.

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About Me

I am an adult onset atheist. I cannot blame parents, society, or an unhappy childhood on my decision to abandon all things theist. I clung for a while to a deist god, but it too was eventually thrown onto the trash-heap. Why insist that I am believing in a “god” when “gravity” or “electromagnetic radiation” are better names? I finally found that I was clinging to the weakest shadow of a deist god because I connected the belief in this imaginary entity with so many good things in the world. One day I realized that those good things would be better without the residue of a belief in god contaminating them.